Which denoiser is better?
For now Intel's OpenImageDenoise is superior to all the other ones. From the release notes:

Compared to the existing denoiser, it works better with more complex materials like glass, and suffers less from splotchy artifacts. It also gives better results with very low numbers of samples, which can be used for quick previews.

It's just a post process so it's easy to set up and thus not exclusive to cycles, means that the node also allows to denoise eevee renders as well as images synthesized by other render engines.

Also see: Type of Denoiser built into Blender